It is generally unacceptable to take an old cable system completely off a pole line before stringing a new cable system on the pole line, since taking the old cable system completely off the pole line generally entails an interruption in communication. Thus, it is generally necessary to keep the old cable system in place, or at least still up on the pole line, until the new cable system is ready to be cut in.
While it is possible to string the new cable system vertically above or vertically below the old cable system, both options are often unattractive. In the first place, there are minimum distances (set by codes, military regulations, or the like) which must be observed between the new cable system and other cable systems (such as power lines and telephone lines) which are strung on the same pole line. Because space is often at a premium on pole lines, the existing cable systems are generally strung at the minimum distances between the cable systems. Thus, a new cable system can only be strung below the cable system it is replacing if the cable system it is replacing is the bottom one on the pole line, and, correspondingly, it can only be strung above the cable system it is replacing if the cable system it is replacing is the top one on the pole line. Moreover, even if the cable system that is being replaced is either the top or the bottom cable system on the pole line, it is often impractical to put the new cable system in above all the old cable systems because there is insufficient pole remaining above the highest previous cable system or below all the old cable systems because that would make the new cable system dangerously low relative to traffic.
Moreover, even where it is possible to string the new cable system vertically above or vertically below the old cable system, doing so necessitates installation of a wood screw threaded hook, which is expensive and time consuming.